Towards the Characterisation of Flow Perturbation Transmission in Convergent Nozzles affecting Free Water Jet Surfaces

Abstract

Free water jets are elementary for many engineering applications, where a convergent nozzle is commonly used to convert the available potential energy into kinetic energy to obtain high-velocity water jets. The free water jet surface can be severely deformed by flow perturbations, e.g., secondary flows induced by flow separation or bends, and cause unwanted free water jet column breakup. Convergent nozzles accelerate the flow and thereby affect the perturbation properties during conveyance. This transmission characteristic is studied by multiphase large eddy simulations, where the free water jet column interacts with a flat surface. The spectral properties of the flow fluctuations are monitored upstream and downstream of the nozzle, where three different nozzle convergence angles are investigated. The simulations reveal that with increased flow acceleration, i.e. higher nozzle convergence angles, the flow experiences relaminarisation. Furthermore, a change in fluctuation properties is observed after the water jet/flat surface interaction. The numerical simulation results are validated against experimental pressure measurements.